CVE-2023-28115 in Snappy
Summary
by MITRE • 03/18/2023
Snappy is a PHP library allowing thumbnail, snapshot or PDF generation from a url or a html page. Prior to version 1.4.2, Snappy is vulnerable to PHAR deserialization due to a lack of checking on the protocol before passing it into the `file_exists()` function. If an attacker can upload files of any type to the server he can pass in the phar:// protocol to unserialize the uploaded file and instantiate arbitrary PHP objects. This can lead to remote code execution especially when snappy is used with frameworks with documented POP chains like Laravel/Symfony vulnerable developer code. If a user can control the output file from the `generateFromHtml()` function, it will invoke deserialization. This vulnerability is capable of remote code execution if Snappy is used with frameworks or developer code with vulnerable POP chains. It has been fixed in version 1.4.2.
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Analysis
by VulDB Data Team • 03/18/2023
The vulnerability identified as CVE-2023-28115 affects the Snappy PHP library, which serves as a utility for generating thumbnails, snapshots, or PDF documents from URLs or HTML pages. This library operates by processing user-provided input to create various document formats, making it a potential attack vector when integrated into web applications. The flaw stems from inadequate validation of input protocols before processing, specifically within the file system operations that occur during document generation. The vulnerability exists in versions prior to 1.4.2, where the library fails to properly sanitize protocol schemes before invoking the file_exists() function, creating a pathway for malicious exploitation.
The technical exploitation of this vulnerability relies on PHP's PHAR deserialization capabilities, which represent a well-documented attack pattern that falls under CWE-502 - Deserialization of Untrusted Data. When an attacker can upload arbitrary files to a server and subsequently manipulate the input to the generateFromHtml() function, they can inject the phar:// protocol scheme into the processing pipeline. This protocol allows PHP to treat uploaded files as PHAR archives, triggering automatic deserialization of the file contents. The deserialization process can instantiate arbitrary PHP objects, potentially leading to remote code execution when combined with vulnerable object propagation (POP) chains found in popular frameworks. This vulnerability directly aligns with ATT&CK technique T1203 - Exploitation for Client Execution, where the attack targets the deserialization mechanism to achieve code execution.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it can enable full system compromise when Snappy is integrated with frameworks that contain documented POP chains such as Laravel or Symfony. The attack surface becomes particularly dangerous when developers use Snappy in conjunction with these frameworks, as the vulnerability can be exploited to execute arbitrary code on the target server. The severity increases when considering that attackers only need to control the output file path of the generateFromHtml() function to trigger the deserialization, making it a critical vulnerability for web applications that process user input through Snappy. The fix implemented in version 1.4.2 addresses this by introducing proper protocol validation before file system operations, preventing the PHAR protocol from being passed directly into functions that could trigger deserialization.
Organizations utilizing Snappy must implement immediate mitigations to protect against this vulnerability, including updating to version 1.4.2 or higher and implementing additional input validation measures. Security teams should also consider restricting file upload capabilities and monitoring for unusual file system access patterns that could indicate exploitation attempts. The vulnerability demonstrates the importance of proper input sanitization and protocol validation in web applications, particularly when dealing with file system operations that could trigger PHP's deserialization mechanisms. This case study highlights the necessity of following secure coding practices and maintaining up-to-date dependencies to prevent exploitation of known vulnerabilities that could lead to complete system compromise.